Electric horns as commonly used on automotive vehicles have traditionally used a vibrating diaphragm driven by an electromagnetic device. Current pulses are developed by a mechanical switch responsive to diaphragm movement such that the switch, being normally closed, would energize a magnetic coil to cause diaphragm movement in one direction against its spring bias and the movement would open the switch allowing the diaphragm return in the other direction thus closing the switch and causing the cycle to repeat. The life of such horns is limited by the life of the mechanical switch used in the horn. It is therefore desirable to devise an alternative to the mechanical switch, however requirements of high power, immunity to high voltage spikes caused by switching an inductive load, and mechanical ruggedness places severe limits on the technology that may be successfully employed.
It has been proposed in the U.S. Pat. No. 3,846,792 to Haigh to use an electronic driver to supply short current pulses to an electric sound-producing device. In that driver an oscillator is used to provide a series of pulses to an electromagnet which attracts a ferromagnetic diaphragm. The pulses have a repetition rate substantially less than the natural frequency (3000 Hz) of the diaphragm. For each pulse, the electromagnet attracts and then releases the diaphragm to allow it to vibrate through a number of cycles before applying another pulse. A feedback circuit responsive to diaphragm position slaves the pulse timing to the diaphragm frequency to assure efficient coupling. This arrangement is adapted to high frequency horns which have small diaphragm movement and readily continue to vibrate when input pulses are removed, and does not apply to low frequency (400-500 Hz) horns. The diaphragms of the low frequency horns do not sustain ringing long after the input pulse is removed. Moreover, the feedback circuit of Haigh is ineffective to accurately time the pulse to the diaphragm movement at low frequency.
To obtain efficiency of operation of a horn, it is necessary to couple the electrical energy into the mechanical part of the system in a manner which makes best use of that energy already imparted to the diaphragm assembly. In the case of a low frequency horn, the synchronism of input pulses and diaphragm movement is of paramount importance in obtaining the highest sound energy output for a given electrical power input. The prior proposal does not provide a solution to attaining that end.